Diaryl iodonium compositions are known antimicrobial agents. Substituted phenyl-thienyl iodonium compositions in particular are effective against bacterial growth, mold, mildew, slime, and viral pathogens. Many substituted phenyl-thienyl iodonium compositions, however, produce undesirable decomposition products such as chlorobenzene and 1,2-dichlorobenzene. Such decomposition products are harmful to mammals including humans.